In the operation of an internal combustion engine, electrical motor, fuel cell system, or other power-producing device or system for a vehicle, waste heat is invariably produced. Such waste heat must be removed from the vehicle in order to endure that the vehicle operates properly. Cooling systems employing one or more liquid coolant loops are often used for such a purpose. Coolant is circulated through various components of the liquid coolant loop, and is placed in heat exchange relation with heat producing sources and/or fluids circulating through the power-producing device or system. By way of example, such heat-producing sources can include engine blocks, batteries, transmissions, and fuel cell stacks, among others. Such fluids can include, by way of example, combustion air, exhaust gas, lubricating oil, transmission oil, fuel, and others.
The waste heat that is transferred to the coolant circulating through the liquid coolant loop must eventually be rejected from the vehicle itself. This step of heat rejection typically occurs by rejecting the heat from the liquid coolant to the ambient air in one or more radiators, thereby maintaining the temperature of the coolant in the liquid cooling loop within a relatively narrow and constant temperature range. While this is the most efficient means by which the waste heat can be removed from the vehicle, it does have its drawbacks. The temperature range within which the liquid coolant loop is controlled must be in excess of the temperature of the ambient air to which the heat is eventually rejected.
While such a limitation has traditionally not been problematic, as vehicle propulsion technologies have progressed, the desirable temperatures to which the heat-producing sources are controlled have reduced, making the take of rejecting the waste heat to a liquid coolant loop more challenging. By way of example, the desirable operating temperature of battery packs for electrical vehicles is substantially below the traditional operating temperatures of internal combustion engines. Similar concerns are raised in the operation of hydrogen fuel cell stacks. In addition, more stringent emissions regulations have driven a demand for reduced temperature levels in fluids such as pressurized combustion air (so-called charge air) and recirculated exhaust gas, among others.
The aforementioned concerns can be addressed by using a refrigerant circulating through a vapor compression cycle to cool the coolant of the liquid coolant loop to a temperature that is below that which can be readily achieved by direct rejection of the waste heat from the coolant to the ambient air. Such an approach can even be used to cool the coolant to a temperature that is sub-ambient, i.e. lower than the actual ambient temperature of the vehicle. While such a system may be suitable for its intended purpose, the operation of a vapor compression refrigerant system to remove the typically large rates of heat production associated with vehicle power sources can lead to undesirable decreases in overall system energy efficiency.
In some particular cases, it may be sufficient to provide a flow of chilled coolant to the heat-producing sources and/or heat-carrying fluids only during relatively short periods of operation where increased cooling capability is especially desirable. By way of example, during short and transient periods of high acceleration a flow of chilled coolant can be provided to produce increased heat transfer. Such short and transient events can be most efficiently accommodated by maintaining a storage of sufficient quantity of chilled fluid on board the vehicle, with the requisite chilling of the coolant being achieved by the refrigerant between successive events.